428 BIOLOGIC RELATIONS BETWEEN PLANTS AND ANTS. 



more advantageous in proportion as they are convenient to the food 

 which the insects seek. Therefore, if a nectariferous plant visited by 

 ants presents in some of its organs a cavity suitable for their habita- 

 tion, it will soon become a lodging for these insects. 



Such is the case with various ferns. "We have already mentioned the 

 extra-nuptial nectaries found on the fronds of various indigenous and 

 exotic ferns. On the lower face of the sterile fronds of Polypodium nec- 

 tar if erum nectaries are found in considerable numbers; but their origin 

 is different from that of those observed in the before-mentioned types. 

 They seem by their position to correspond to the petiolar nectaries of 

 phanerogams of the types already cited. But in P. nectariferum they 

 appear to be aborted sori formed at the points where the nervures 

 divide, and therefore analogous to the floral nectaries of the phanero- 

 gams. The sterile frond of this polypod is quite different in shape from 

 the fertile frond. The nectaries seem to attract the ants to it, and they 

 find there assured shelter by reason of its special form. 



The young shoots of palm trees are tender, usually sweet in flavor, 

 and therefore exposed to be eaten by herbivorous animals. (It is well 

 known that travelers who traverse the virgin forests of Malasia easily 

 procure for themselves a succulent food by felling palm trees and cut- 

 ting out their growing top shoots.) These plants are therefore usually 

 protected by means of spines. Certain species for which this mode of 

 defense is insufficient have recourse to ants for protection. Even those 

 species that are armed with sharp needles have the younger parts com- 

 paratively unprotected, because the needles are not yet sufficiently 

 hardened. The ants find a shelter upon these palm trees. Sometimes, 

 as is the case with certain species of Calamus, the spathe that protects 

 the inflorescence has a form suitable for harboring these insects. Some- 

 times, as in some species of Dwmonor ops, the series of needles that arm 

 the stem are curved toward each other two by two, thus forming, by 

 their intercrossing on the surface of the stem, galleries, in which the 

 ants establish themselves. In this case the lodging organ forms but a 

 part of the wall of the cavity inhabited by the ants. It is, as might 

 be said, the rough draft of a myrmecophilous feature. In the great 

 majority of lodging plants the cavity is entirely formed by the organs 

 of the plant. 



In palm trees of the genus Korthalsia the lodging organ is of 

 another character. The sheath of the leaves (ocrea) has an appendage 

 that enlarges in the form of a boat, and thus shuts in, together with that 

 part of the stem against which it is applied, a closed cavity. To get 

 into this the ants make an opening in the median line or laterally, and 

 in addition to this, which is used for entrance and exit, they make other 

 small openings at the base of the ocrea for the purpose of ventilation. 

 It is probable that the Korthalsia has nectaries in the petiolule of some 

 of the segments of its leaves. It appears certain that the cavity of the 

 ocrea is formed without any intervention on the part of the ants, but 



